The present invention relates to a nonvolatile semiconductor memory and a process for fabricating the same, and more specifically to a nonvolatile semiconductor memory having a reduced chip area and a process for fabricating the same.
Referring to FIG. 1, there is shown a diagrammatic plan view illustrating a prior art nonvolatile semiconductor memory. FIG. 2 is a diagrammatic sectional view taken along the line Dxe2x80x94D in FIG. 1, and FIGS. 3A and 3B are views similar to that of FIG. 2 but showing different problems in the prior art nonvolatile semiconductor memory.
In FIGS. 1, 2, 3A and 3B, Reference Number 51 designates a floating gate of a memory cell in the nonvolatile semiconductor memory, and Reference Number 52 denotes a control gate formed above the floating gate 51 and functioning as a word line. Reference Number 53 indicates an insulating film between the floating gate 51 and the control gate 52. Reference Numbers 54 and 55 show a drain and a source of the memory cell, respectively, and Reference Number 56 designates a channel region between the drain 54 and the source 55. Thus, one memory cell is formed by the drain 54, the source 55, the channel region 56 between the drain 54 and the source 55, and the floating gate 51 and the control gate 52 stacked above the channel region 56, and each memory cell is isolated by a device isolation region 57 from adjacent memory cells.
Reference Number 61 indicates a selection transistor (selector) for selecting the memory cells. The selection transistor 61 is controlled by a selection signal line 62 functioning as a gate electrode.
The nonvolatile semiconductor memory thus formed includes a memory cell region in which a number of memory cells are formed and a selection transistor region in which a plurality of selection transistors are formed. The memory cell region is adjacent to the selection transistor region, but since the memory cells and the selection transistors cannot be formed in completely the same process, when the memory cells are formed, the selection transistor region is masked, and when the selection transistors are formed, the memory cell region is masked. Therefore, because of a misalignment of the mask, an unetched portion 64 remains at a boundary between the memory cell region and the selection transistor region, as shown in FIG. 3A, and in a later process, the unetched portion 64 collapses, with the result that the yield of production is deteriorated.
Alternatively, because of the misalignment of the mask, a substrate is overetched at the boundary between the memory cell region and the selection transistor region, as shown with Reference Number 65 in FIG. 3B by two etchings, one of which is performed for forming the memory cells, and the other of which is performed for forming the selection transistors.
In order to overcome the above problems, the prior art nonvolatile semiconductor memory was so constructed to have a dummy gate line 66 between the selection transistor region and the memory cell region, as shown in FIGS. 1 and 2. However, if the dummy gate line 66 is provided, the chip area of the nonvolatile semiconductor memory inevitably becomes increased.
Accordingly, it is an object of the present invention to provide a nonvolatile semiconductor memory which has overcome the above mentioned problem of the conventional prior art.
Another object of the present invention is to provide a nonvolatile semiconductor memory having a reduced chip area by making the dummy gate line unnecessary, and the process for fabricating the same.
Still another object of the present invention is to provide a nonvolatile semiconductor memory including the selection signal line having the function of the dummy gate line, thereby to reduce a necessary chip area, and the process for fabricating the same.
The above and other objects of the present invention are achieved in accordance with the present invention by a nonvolatile semiconductor memory comprising:
a memory cell region including a number of memory cells formed therein and each having a floating gate and a control gate formed above the floating gate, and a plurality of word lines extending in a first direction in parallel to each other, separately from each other; and
a selection transistor region positioned adjacent to the memory cell region and including one selection transistor formed therein and a selection signal line extending in parallel to the word lines;
wherein the selection signal line is formed on an inactive region in a substrate to extend in parallel to a boundary line between the memory cell region and the selection transistor region.
In an embodiment of the nonvolatile semiconductor memory, the inactive region includes a device isolation region formed in a principal surface of the substrate and a thick oxide film covering a diffused region formed in the principal surface of the substrate. For example, the selection signal line includes a layer which is formed of the same material as that of the control gate, and extends in parallel to the boundary line between the memory cell region and the selection transistor region, at a position retracting from the boundary line between the memory cell region and the selection transistor region.
According to another aspect of the present invention, there is provided a nonvolatile semiconductor memory comprising;
a memory cell region including a number of memory cells formed therein and each having a floating gate and a control gate formed above the floating gate, and a plurality of word lines extending in a first direction in parallel to each other, separately from each other; and
a selection transistor region positioned adjacent to the memory cell region and including one selection transistor formed therein and a selection signal line extending in parallel to the word lines;
wherein the selection signal line includes a first layer which is formed of the same material as that of the floating gate and a second layer which is formed of the same material as that of the control gate.
Preferably, the selection signal line includes a short selection signal line extending therefrom in a second direction orthogonal to the first direction and going apart from the memory cell region, an end of the short selection signal line constituting a gate of the selection transistor.
In addition, the selection signal line includes a side surface extending on and along the boundary line between the memory cell region and the selection transistor region.
In a preferred embodiment, the selection signal line includes a short selection signal line extending therefrom in a second direction orthogonal to the first direction and going apart from the memory cell region, an end of the short selection signal line constituting a gate of the selection transistor.
According to a third aspect of the present invention, there is provided a process for fabricating a nonvolatile semiconductor memory which has a memory cell region including a number of memory cells formed therein and each having a floating gate and a control gate formed above the floating gate, and a plurality of word lines extending in a first direction in parallel to each other, separately from each other, and a selection transistor region positioned adjacent to the memory cell region and including one selection transistor formed therein and a selection signal line extending in parallel to the word lines, the processing comprising the steps of:
forming a device isolation film on a principal surface of a semiconductor substrate, forming a tunnel insulator film for the memory cells on the principal surface of the semiconductor substrate, forming a floating gate film on the tunnel insulator film, partially patterning the floating gate film, and forming source/drain regions at opposite sides of the partially patterned floating gate film by using the partially patterned floating gate film as a mask; forming an oxide film to cover the whole surface of the semiconductor substrate, and etching back the oxide film until an upper surface of the partially patterned the floating gate film;
removing the partially patterned floating gate film from the selection transistor region;
forming on the whole surface of the semiconductor substrate an insulating film for insulating between the floating gate and the control gate, and removing the insulating film from the selection transistor region;
forming a gate insulator film for the selection transistor;
forming a polysilicon film on the whole surface of the semiconductor substrate;
selectively etching the polysilicon film and the partially patterned floating gate film under the polysilicon film within the memory cell region to form a control gate and a underlying floating gate while etching one side surface of the selection signal line, extending on and along a boundary line between the memory cell region and the selection transistor region; and
selectively etching the polysilicon film within the selection transistor region to form a gate of the selection transistor while etching the other side surface of the selection signal line.
According to a fourth aspect of the present invention, there is provided a process for fabricating a nonvolatile semiconductor memory which has a memory cell region including a number of memory cells formed therein and each having a floating gate and a control gate formed above the floating gate, and a plurality of word lines extending in a first direction in parallel to each other, separately from each other, and a selection transistor region positioned adjacent to the memory cell region and including one selection transistor formed therein and a selection signal line extending in parallel to the word lines, the processing comprising the steps of:
forming a device isolation film on a principal surface of a semiconductor substrate so that the device isolation film extends over a boundary region between the memory cell region and the selection transistor region, forming a tunnel insulator film for the memory cells on the principal surface of the semiconductor substrate, forming a floating gate film on the tunnel insulator film, partially patterning the floating gate film, and forming source/drain regions at opposite sides of the partially patterned floating gate film by using the partially patterned floating gate film as a mask;
forming an oxide film to cover the whole surface of the semiconductor substrate, and etching back the oxide film until an upper surface of the partially patterned the floating gate film;
removing the partially patterned floating gate film from the selection transistor region;
forming on the whole surface of the semiconductor substrate an insulating film for insulating between the floating gate and the control gate, and removing the insulating film from the selection transistor region;
forming a gate insulator film for the selection transistor;
forming a polysilicon film on the whole surface of the semiconductor substrate;
selectively etching the polysilicon film and the partially patterned floating gate film under the polysilicon film within the memory cell region to form a control gate and a underlying floating gate by using a mask substantially completely covering the selection transistor region and having a side surface which slightly retracts from the boundary line between the selection transistor region and the memory cell region and which is still positioned on the device isolation region; and
selectively etching the polysilicon film within the selection transistor region to form a gate of the selection transistor by using a mask substantially completely covering the memory cell region and having a side surface which slightly retracts from the boundary line between the selection transistor region and the memory cell region and which is still positioned on the device isolation region.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.